\documentclass[12pt]{article}
\usepackage{fontspec}
\setmainfont[Script=Arabic]{XB Niloofar}
\usepackage{bidi}
\def\myoalign#1{\leavevmode\vtop{\baselineskip0pt
\lineskip.1ex%
\ialign{##\crcr#1\crcr}}}
\def\mkasre#1{{\myoalign{\char"200D#1\char"200D\protect\crcr%
\hidewidth\hss ِ\hss\hidewidth}}}
\def\bletter#1{\char"200D#1}
\def\aletter#1{#1\char"200D}
\begin{document}
\setRTL
\aletter{سُ}\mkasre{یّ}\bletter{رَ}ت

\bigskip
سُیِّرَت
\end{document}